1. Field of the Invention
The present invention relates to a carbon fiber and method for fabricating the same, and in particular relates to a high module carbon fiber and method for fabricating the same.
2. Description of the Related Art
Carbon fibers have advantages of low specific gravity, good mechanical properties (tensile strength and module), high electric and thermal conductivity, and good knittability. Carbon fibers with high module and high strength are commonly used as reinforcement materials in advanced structural composites for building, navigation, aircraft or military applications. Raw materials of carbon fibers can be rayon, polyvinyl alcohol, polyvinylidene chloride, polyacrylonitrile (PAN), or pitch. Currently, carbon fibers are generally prepared from polyacrylonitrile (PAN) as raw material to fabricate carbon fibers with desired tensile strength. The graphite crystal characteristics of PAN carbon fibers are determined by XRD and Raman spectroscopy.
In XRD analysis of carbon fiber, the crystalline stacking size Lc of the graphite layer (indicating the <002> crystal orientation)) is determined by the half-width of the diffraction peak β, as described by the Equation (I):Lc=Kλ/β cos θ  Equation (I)
K: constant; λ: wavelength of x-ray; θ: diffraction angle
The compactness of carbon fiber is proportional to the crystalline stacking size Lc thereof. Namely, the carbon fibers with higher crystalline stacking size Lc would exhibit improved tensile module.
In Raman analysis, a R is defined as a background-free Raman spectral intensity area ratio D/G of a G-peak appearing at wavelength of about 1580 cm−1 and a D-peak appearing at wavelength of 1350 cm−1, as described by Equation (II):R=D/G  Equation (II)
The G-peak results form the lattice vibrations of sp2 bonding in the graphite sheet and the d-peak results from the vibrations of carbon atoms located at the graphite sheet edge (defective graphite structure). The R value reduces in inverse ratio to the graphitization degree. Further, the R value has a relationship with the crystalline planar size La as shown in Equation (III)La=44×R−1  Equation (III)
In theory, the carbon fiber with higher crystalline planar size La exhibits improved graphitization degree, and increased grain size, but increased planar size along the fiber axis results in reducing tensile strength.
As shown in Table 1, the crystalline stacking size Lc and the crystalline planar size La of the carbon fibers (Toray-T300) are proportional to the graphitization temperature (from 2400° C. to 3000° C.). Further, the tensile modulus is proportional to the crystalline stacking size Lc, but the tensile strength is in inverse proportion to the larger crystalline planar size La.
TABLE 1tensileProcessmodulus/tensile strength/temperatureLc (Å)La (nm)GPaGPa240040.914.673433.14250044.815.203562.85260046.516.183622.82270053.217.363812.66280058.318.213912.5290062.919.114182.24300068.419.654242.2
PAN carbon fibers generally have high tensile strength. However, due to the chaotic crystalline stacks, PAN carbon fibers exhibit inferior tensile module. In order to fabricate high tensile strength and high module PAN carbon fibers, a graphitization process with a higher process temperature and a longer graphitizing period is called for. Due to the low cost, the high tensile strength PAN carbon fiber has become mainstream in recent years, in comparison with commercial high tensile strength and high module carbon fiber.
On the other hand, due to the higher crystalline planar size La, the high tensile strength and high module carbon fiber (Toray MJ series) exhibits lower tensile strength than the high tensile strength carbon fibers (Toray T series). In conventional graphitization processes, the crystalline stacking size Lc and the crystalline planar size La are increased simultaneously. However, the carbon fiber has higher crystalline planar size La resulting in lower tensile strength.
It is important to reduce the fabrication cost of the high tensile strength and high module carbon fiber. In the convention graphitization process, the obtained carbon fiber has a tensile module proportional to the graphitization temperature, but has a tensile strength in inverse proportion to the graphitization temperature. It is necessary to improve the tensile modulus of the high tensile strength PAN carbon fibers without reducing the tensile thereof. Specifically, the crystalline stacking size of the high tensile strength carbon fiber should be increased based on the premise that the crystalline planar size La is not greatly changed, in order to fabricate high tensile strength and high module carbon fibers.
There are several graphitization processes for fabricating carbon fibers such as graphitization employing a conventional electric furnace, as disclosed in JP200780742, TW 561207, TW 200902783, and TW279471. Those patents disclosed the methods for fabricating carbon fibers via an electric furnace. However, due to the low thermal conductivity, incomplete thermal insulation and low heating rate, the total graphitization process by means of an electric furnace has a process period of 1-10 hr. Therefore, it is hard to limit the crystalline planar size La within a specific range. The aforementioned graphitization process is very time-consuming and power-consuming. Thus, its use is disadvantageous in view of the cost of carbon fibers.
Moreover, a graphitization process in company with microwave induction heating has been developed and includes the following steps. Fibers prepared from pitch, coal, or fibrin are subjected to a pre-graphitization process (t a temperature of more than 300° C., such as 300-1500° C.). Next, the-graphitization fibers are subjected to a graphitization process with microwave induction heating. The aforementioned process has a disadvantage of requiring a longer pre-graphitization period (of more than 4 hr). Further, since the raw materials used in the process (such as pitch, coal, or fibrin) have a low carbon content, it is hard to fabricate high strength and high module carbon fibers via this method.
U.S. Pat. No. 6,372,192 B1 discloses a graphitization process of polyacrylonitrile fiber (PAN) with microwave plasma, including subjecting a PAN fiber to a pre-oxidization at 500° C., and performing the graphitization process with microwave plasma to the pre-oxidized carbon fiber under vacuum. Since the microwave energy transmitted by gas ions only achieves the outward portion of the pre-oxidized carbon fiber and the generated heat is difficult to conduct into the inward portion of the pre-oxidized carbon fiber, the obtained fiber exhibits low tensile strength (2.3 GPa) and low tensile module (192 GPa).
Therefore, it is necessary to develop a novel polyacrylonitrile carbon fiber with higher crystalline stacking size Lc and lower crystalline planar size La compared to conventional carbon fibers. The module will be enhanced (more than 200 GPa) and will meet the increased tensile strength requirements.